A lithography apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithography apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In an immersion lithography apparatus, a liquid is confined to an immersion space by a liquid confinement structure. The immersion space is between a final optical element of a projection system through which the pattern is imaged and the substrate onto which the pattern is transferred or a substrate table on which the substrate is held. The liquid may be confined to the immersion space by a fluid seal. The liquid confinement structure may create or use a flow of gas, for example to help in controlling the flow and/or the position of the liquid in the immersion space. The flow of gas may help form the seal to confine the liquid to the immersion space.
Defects in the pattern applied to the substrate are undesirable as they reduce the yield, i.e. the number of usable devices per substrate. Because many patterning steps are required to make a device, even a very low rate of defects per exposure can significantly reduce the yield. There are two types of defect that are peculiar to an immersion lithography apparatus.
A droplet of liquid or a liquid film (hereinafter reference to a droplet also encompasses a film; a film being a droplet covering a larger surface area) from the immersion space may be left on the substrate after exposure of a target portion. If the droplet is in contact with the resist for a significant period it can degrade the resist by leaching. If the droplet evaporates, it can leave behind debris and/or may induce a localised cooling. Defects resulting from droplets left on the substrate, whether by resist degradation or evaporation, are referred to herein as trail defects.
A second form of defect peculiar to an immersion lithography apparatus occurs if a bubble is formed in the immersion liquid. If the bubble moves into the path of a projection beam used to project the image of the patterning device onto the substrate, then the projected image will be distorted. Defects caused by bubbles are referred to herein as exposure defects.
Trail defects and exposure defects may cause problems for objects other than the substrate, such as sensors.